Gordon I use stainless steel mesh in all my shrimp tanks and its completely shrimp-safe. I don't buy the fancy 'moss ties/string" that ADA sells- I just use regular cotton threads for sewing and they breakdown just fine with time.
If you go to Home Depot or other local hardware store, you can buy the screen mesh they sell for screen doors. There are different hole sizes. Then it it to twice the length you want or twice the width. Then you place your moss or ricca inside, fold and thread the sides together = moss or riccia sandwich that is inert, cheap, and will stay down. I started doing this with what little riccia i had and it's growing nicely.